In the field of arc welding, the three (3) main types of arc welding are submerged arc welding (SAW), shielded metal arc welding (SMAW), and flux-cored arc welding (FCAW). In submerged arc welding, coalescence is produced by heating with an electric arc between a bare-metal electrode and the metal being worked. The welding is blanketed with a granular or fusible material or flux. The welding operation is started by striking an arc beneath the flux to produce heat to melt the surrounding flux so that it forms a subsurface conductive pool which is kept fluid by the continuous flow of current. The end of the electrode and the work piece directly below it become molten and molten filler metal is deposited from the electrode onto the work. The molten filler metal displaces flux pool and forms the weld. In shielded metal arc welding, shielding is by a flux coating instead of a loose granular blanket of flux. In flux-cored electrodes, the flux is contained within the metal sheath.
In the art of welding, much prior effort has been expended in developing flux compositions of the type having predetermined flux components intended to perform in predetermined manners. A large number of compositions have been developed for use as fluxes in arc welding both for use generally as welding fluxes and for use as a coating on a metallic core or within a sheath. Fluxes are utilized in arc welding to control the arc stability, modify the weld metal composition, and provide protection from atmospheric contamination. Arc stability is commonly controlled by modifying the composition of the flux. It is therefore desirable to have substances which function well as plasma charge carriers in the flux mixture. Fluxes also modify the weld metal composition by rendering impurities in the metal more easily fusible and providing substances which these impurities may combine with in preference to the metal to form slag. Practically all slag-forming compounds may be classed as either acidic or basic, according to which compounds they react with. The substances which are considered to be the most active “bases” are those which are compounds of the elements forming basic compounds in ordinary chemical reactions in water solutions, such as calcium, magnesium, and sodium. The most active “acid” impurities are compounds of silicon, titanium, zirconium and aluminum. Fluxes are prepared with a higher or lower percentage of acidic or basic compounds, depending on the type of metal to be welded and impurities in the metal. In some instances, other materials may be added to lower the slag melting point, to improve slag fluidity, and to serve as binders for the flux particles.
One problem encountered in the welding industry is the absorption of moisture by the flux covering on welding electrodes. During welding, the heat evaporates and dissociates the water, evolving hydrogen gas, which can dissolve into the metal. Under stress, the dissolved hydrogen in the weld metal may produce cracks with the potential for catastrophic failure of the weld. Hydrogen embrittlement is a phenomenon which involves loss of ductility and increased crack susceptibility in steel at room temperature due to the presence of hydrogen in the steel. Hydrogen induced cracking can occur to some extent whenever sufficient hydrogen and stress are present in a hard steel at temperatures above −100° C. and below 150° C. As it is almost impossible to avoid producing these stresses in a weld. Methods of crack control usually involve controlling the amount of hydrogen present in the weld, the microstructure of the solidified weld metal, or both. Hydrogen can be introduced into the weld arc atmosphere from a number of sources including oxides, wire contaminants and oil. The primary source of hydrogen is moisture in the flux and flux binder.
Binders are used in granular fluxes and in electrode coatings to hold the components of the flux system together and/or to maintain the desired shape of the electrode coating about the metallic core during normal handling. Most welding flux formulations consist of an oxide-based material (flux) and additives bonded together by sodium silicate and/or potassium silicate (water glass). These types of binders are disclosed in U.S. Pat. Nos. 4,103,0677; 4,131,784; 4,208,563; 4,355,224; 4,741,974 and 5,300,754, all which are incorporated herein by reference. Such binders have been particularly useful because they resist decomposition under conditions of use and because such binders provide adequate strength characteristics in the quantity added to the flux composition for the high rate of extrusion used in the manufacture of electrodes. In addition, the specific properties of either potassium silicate or sodium silicate makes each attractive for the manufacture of welding electrodes. For example, the drying characteristics are such that the liquid silicates used as binders for coating metal electrodes become hard films through the loss of water. The use of silicates in the flux can enhance arc stability during welding. The silicates in the flux provide a low melting point component to the flux which facilitates in adjusting the melting/freezing range of the slag. Silicates are easy to handle and use, thus making desirable for use as flux binders. Silicates are also relatively inexpensive, thus adding little cost to the flux composition. Sodium and potassium silicates have been particularly useful because their properties provide characteristics which are desirable in the manufacture of coated electrodes. With the addition of liquid sodium silicate to a dry powder formulation, the resulting mixture can be kneaded to a consistency that is appropriate for subsequent extrusion. The mass of kneaded mixture is typically formed into “slugs” which facilitates in handling during the time of storage and the loading of presses with the mixture for the extrusion operation. At present, a substantial portion of commercially produced coated electrodes are produced by the extrusion process. The plasticity of the flux coating on the wire electrode is somewhat controlled by the silicate addition in the flux mixture, but may also be influenced by other ingredients such as raw clay or bentonite which may be added or combined with silica or calcined clay. As the electrode is extruded, the electrode becomes reasonably solid and resists flattening as soon as the electrodes leave the die and falls on a conveyor belt. Drying of the extruded flux coating on the wire electrode is carried out at a low temperature beginning at about 100-150° C. with controlled humidity in order to obtain uniform drying of the flux coating without cracking. This drying step is followed by one or more higher temperature drying steps at a lower humidity depending upon the nature of the flux coating. The moisture content of the dried flux coating on the electrode will range from less than 0.2% in some low hydrogen electrodes to as high as 3 to 6 percent in a cellulose type of electrode (e.g. E6010, E6011, etc.).
In high strength, low hydrogen electrodes, sodium silicate and/or potassium silicate binders have not been very satisfactory. Sodium silicate and/or potassium silicate binders are very hygroscopic and require moisture to keep them sound and free from cracks. During welding, the heat evaporates and dissociates the water, evolving hydrogen gas which can dissolve into the weld metal. Under stress, the dissolved hydrogen can produce cracks in the weld metal. The amount of moisture retained by silicate and/or potassium silicate binders is governed primarily by the temperature to which it has been dried. It is known that room temperature air-drying of the silicate is not adequate for films or bonds that are to be used in welding. In an effort to decrease the possibility of failure, the presently available welding electrodes are baked at 370-540° C. or greater to decrease the water in the flux to less than 0.2%. The maintenance of this degree of dryness has been important in the welding of higher strength materials, and such maintenance necessitates careful handling to avoid hygroscopic moisture pickup during the use of these electrodes. Although moisture pickup has not been particularly troublesome in coatings for lower strength weld metal, the hygroscopic characteristics of the present day low hydrogen coatings has made it mandatory to use heated ovens to maintain the dryness of the flux coating to restrict the pick up of moisture. For high strength welds, the hygroscopic nature of the silicates in the flux coatings has been particularly damaging since, for example, in the EXX18 type of electrodes, the moisture content must be kept at a level below 0.2 percent. As a result, these electrodes can only be used for a limited time before the fluxes absorb moisture from the air and thus have to again be baked to reduce the moisture content. Some in the art are of the opinion that low hydrogen electrodes can not be successfully rebaked at low temperatures, to sufficiently reduce the moisture content of the flux coating. As such, some skilled in the art are of the opinion that the most appropriate way to avoid hydrogen absorption by the weld metal is to keep the moisture content of the flux coating to a minimum after being initially dried. As a result, stringent controls have been placed on the moisture levels of the low hydrogen electrode.
A problem in addition to that of water absorption by these weld fluxes, especially submerged arc fluxes, is their lack of CO2 containing compounds during welding. Compounds containing CO2 are added to some submerged arc fluxes to generate CO2 during welding. These CO2 containing compounds can enhance the operability of the flux by increasing the stability of the arc and by excluding or reducing atmospheric contamination, particularly N2, from the weld metal. Baking the flux at temperatures about 540° C. tends to decompose sources of CO2 in the flux, such as calcium carbonate. As a result, the drying times for the flux composition has to be reduced so as not to expel the CO2 from the flux when a certain CO2 content is desired; however, the reduced drying time results in increased moisture content of the flux, and less setting time for the flux.
Another problem with baking the fluxes at 540° C. or greater to remove moisture is that several flux components, such as metallic powders, which may be added to provide alloying of the weld metal, will oxidize during the baking operation (e.g. Al, Mg, Ti, etc.). Metal powders are added as required for alloying with the various types of metals or steels to be welded. Fluorides may be added to lower the viscosity of the flux at operating temperature thereby altering the fluidity of the molten flux on the steel. Flexibility in choosing the type of metal powders to be added to the flux rather than changing the composition of the steel rod decreases the cost of the welding electrodes. In addition, some materials that are included in the flux system react with the silicate at high temperatures (e.g., kaolin, etc.).
Several flux binders have been developed to address the problems associated with sodium silicate and/or potassium silicate binders. Two of these binders are disclosed in U.S. Pat. Nos. 4,103,067 and 4,662,952, both of which are incorporated herein by reference. The '952 patent discloses a welding flux binder hydrolyzed and polymerized from a mixture of tetraalkylorthosilicate, Si(OR)4, wherein R is —CH3, —C2H5 or —C3H7, alkali and alkaline earth salts. The welding flux made with this binder comprises an alkali-alkaline earth silicate, M2O.M′O.SiO2, wherein M is lithium, sodium, potassium, or other element in Group I of the Periodic Table and M′ is magnesium, calcium, barium, or other element in Group II of the Periodic Table and may further comprise metal compounds. Tetraalkylorthosilicate is an organometallic precursor to a ceramic binder. The organic portion is removed during processing of the weld flux binder and is not present in the final product. Unlike sodium silicate and/or potassium silicate binders, the binder contains a homogeneous distribution of alkali and alkaline earth ions and is not hygroscopic. This is a result of the use of tetraalkylorthosilicate and the presence of compounds which react to form CaO, MgO, BaO, or other alkaline earth oxides. The oxide compounds, particularly calcium compounds, act as stabilizing agents and make the fired binder non-hygroscopic. Alkali compounds, particularly potassium, significantly reduce the viscosity of the glass, lowering the temperature required to sinter the binder to about 500-1100° F. The '067 patent discloses a binder of hydrolyzed organic silicate (such as ethyl silicate) which makes no substantial contribution to the moisture level and which makes the covering resistant to hygroscopic moisture pickup prior to welding. Hydrolyzed ethyl silicate binder reduces the hygroscopicity of the flux. The hydrolyzed organic silicate binder can be used either as a replacement or a supplement for the sodium silicate or potassium silicate binder. As a result, with proper drying in an inert gas protected atmosphere up to 537° C., the hydrolyzed ethyl silicate converts to silica with no moisture which results in a lower moisture content for the flux. In addition, the pickup of moisture by the binder is restricted so that these electrodes may be used for longer periods than ordinary shop practice without excess pick up of moisture.
Although these binders have addressed, some of the moisture pickup problems associated with sodium silicate and/or potassium silicate binders, the binder disclosed in the '952 patent still requires high temperatures to set the binder. The binder disclosed in the '067 patent, though having a lower set temperature, introduces hydrogen in the flux by the use of an organic binder. Such added hydrogen can be detrimental when attempting to obtain extremely low hydrogen levels in the weld metal. In view of the prior art binders, there remains a need for a binder that has low hygroscopicity, a low set point temperature and which does not introduce hydrogen to the flux system.